The present invention relates to a fastening structure having a screw with outer helical grooves, an associated nut with corresponding inner helical grooves, and bearing balls interposed in the nut and screw grooves, where the screw and nut remain engaged even after loss or removal of the bearing balls.
A ball screw assembly is a relatively well known mechanism for converting rotary motion into axial movement, as disclosed in U.S. Pat. No. 1,831,080. As illustrated in FIGS. 1A-1C, a standard ball assembly has a screw 10 with a thread 20, a nut 30 positioned around the screw and internally threaded with a thread 40 having the same pitch as the screw thread 20, and a multitude of bearings 60 disposed in a channel 70 defined between the screw 10 and nut 30 by the screw and nut threads 20 and 40. The channel 70 may be shaped to match closely the shape and dimensions of the bearings 60. The ball screw assembly further includes a return tube 80 in the nut 30 for transferring the bearings 60 between the opposite ends of the nut. In operation of the ball nut assembly, counter-clockwise rotation of the screw causes the bearings 60 to leave the channel 70 at a distal end 31 of the nut 30 and travel via the return tube 80 to the proximal end 32 nut where the bearings 60 are returned into the channel 70. In order to direct the cycling of the bearings 60, the nut may have a structure, such as a finger (not illustrated) that extends into the channel 70 to direct the bearings 60 in to the return tube 80.
The structure of the ball screw assembly allows rotation of the screw 10 relative to the nut 30 to cause precise axial motion of the screw 10 relative to the nut 30. The ball screw assembly has very high mechanical efficiencies and can bear large loads, which make it possible to obtain very high yields and to shift heavy loads using a very low torque input. Because of these properties, the ball screw assembly is frequently employed in moving and manipulating heavy loads. In addition, the ball screw assembly is wear resistant and exhibit very little play. Accordingly, the ball screw assembly is also well-suited for use in precision mechanisms and machine tools employed in commercial, manufacturing, and industrial applications.
A well-known apparatus for mechanically rotating the ball screw assembly is illustrated in FIG. 2. In that figure, a housing, generally designated 1, is provided for the ball screw 10 which is extensible from the housing 1. Received on the ball screw 10, is the nut 30, which circulates the bearings 60 in a recirculating path in the usual, above-described manner. The nut 30 may be driven in rotation by a pinion gear 3 via a worm gear 4 which is in mesh with it. The worm gear shaft 5 may be driven by a motor drive system (not shown). Provided on the nut 30 is an integrated base flange 8 which is received within and coupled to the pinion gear 3.
In operation, rotation of the worm shaft 5 drives the nut 30 in rotation via the worm 4 and pinion 3. With rotation of the ball nut 30, the bearings 60, which travel in the complemental internal threads 40 and external threads 20 of the nut 30 and ball screw 10 respectively, move the screw 10 in axial motion up and down as desired.
Unfortunately, the traditional ball screw assembly has the problem of occasional catastrophic failure. In particular, the balls 60 start to fatigue and deteriorate when used for many cycles, depositing debris in the channel 70 and the return tube 80. This presence of the debris causes the bearings 60 to deteriorate even further. Eventually, the bearings 60 may become so worn and distorted that they are no longer able to prevent separation of the screw 10 from the nut 30. Alternatively, the debris from the deterioration of the bearings 60 collects and blocks the return tube 80, such that the bearings 60 leave the ball screw assembly instead of returning the beginning of the channel 70.
In either case, catastrophic failure occurs as the screw 10 separates from the nut 30 with little resistance. As seen in FIG. 1D, the traditional ball screw assembly has no structural feature to prevent the screw 10 from easily separating from the nut 30 after the bearings 60 are removed from the channel 70. The screw and nut may violently separate, causing the load supported by the ball screw assembly to be abruptly released and dropped, potentially damaging the contents of the load. This failure of the ball screw assembly is sudden and without warning, potentially occurring almost immediately upon the loss of the bearings 60.
Accordingly, it is a goal of the present invention to provide an improved ball screw assembly that is resistant to catastrophic failure and the resulting sudden separation while preserving the ball screw""s benefits of high mechanical efficiencies. A further goal of the present invention to provide a ball screw assembly that gives an indication of potential failure, thereby allowing preventive measures such as repair or replacement of the ball-screw assembly.
These and other goals are addressed through the fail-safe ball screw assembly of the present invention. The ball screw assembly has a double start screw with a first helix with a truncated, smaller diameter flat and an intertwined second helix with an extended, larger diameter flat. The assembly also has corresponding nut with a first helix with an extended, larger diameter flat and an intertwined second helix with a truncated, smaller diameter flat. When the screw is inserted with the nut, the screw""s truncated first helix is paired with the nut""s extended first helix. Likewise, the screw""s extended second helix is paired with the nut""s truncated second helix. The nut and screw helixes combine to form channels in which bearings balls travel. With this structure, the fail-safe ball screw assembly operates with the efficiency and precision of a standard ball screw, while catastrophic failure caused by the removal or loss of the bearing balls is prevented. In particular, the extended, larger diameter flats of the second screw helix and the first nut helix interact, similar to threads in standard screw/nut combinations, to prevent the unintended separation of the screw and nut. When contact occurs between the larger diameter flats of the second screw helix and the first nut helix, the resulting friction diminishes the ball screw assembly""s efficiency, thereby requiring more energy to rotate the ball screw relative to the nut. To take advantage of this indication of deterioration and failure of the ball screw assembly, another implementation of the present invention connects a torque sensor to the ball screw assembly to detect any increase in force needed to turn the ball screw.